CN115136358A - Electrode for lithium ion secondary battery and lithium ion secondary battery - Google Patents

Electrode for lithium ion secondary battery and lithium ion secondary battery Download PDF

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Publication number
CN115136358A
CN115136358A CN202080097315.6A CN202080097315A CN115136358A CN 115136358 A CN115136358 A CN 115136358A CN 202080097315 A CN202080097315 A CN 202080097315A CN 115136358 A CN115136358 A CN 115136358A
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Prior art keywords
electrode
secondary battery
ion secondary
lithium ion
oxide solid
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马场健
松本和明
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Honda Motor Co Ltd
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Honda Motor Co Ltd
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    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
    • H—ELECTRICITY
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    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
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    • H—ELECTRICITY
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    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/052—Li-accumulators
    • H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
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    • H01M10/056—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes
    • H01M10/0561—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes the electrolyte being constituted of inorganic materials only
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    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/056—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes
    • H01M10/0564—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes the electrolyte being constituted of organic materials only
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    • H01M10/058—Construction or manufacture
    • H01M10/0585—Construction or manufacture of accumulators having only flat construction elements, i.e. flat positive electrodes, flat negative electrodes and flat separators
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    • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
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    • H01M4/00—Electrodes
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    • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
    • H01M4/131—Electrodes based on mixed oxides or hydroxides, or on mixtures of oxides or hydroxides, e.g. LiCoOx
    • H—ELECTRICITY
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    • H01M4/36—Selection of substances as active materials, active masses, active liquids
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    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/362—Composites
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    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
    • H01M4/485—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of mixed oxides or hydroxides for inserting or intercalating light metals, e.g. LiTi2O4 or LiTi2OxFy
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    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/58—Selection of substances as active materials, active masses, active liquids of inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy; of polyanionic structures, e.g. phosphates, silicates or borates
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    • H01M2300/0068—Solid electrolytes inorganic
    • H01M2300/0071—Oxides
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Abstract

The purpose of the present invention is to provide an electrode for a lithium ion secondary battery that can satisfy both thermal stability and durability, and a lithium ion secondary battery using the same. Specific electrolyte and highly dielectric solid particles are present on the electrode composite layer. Specifically disclosed is an electrode for a lithium ion secondary battery, which is provided with an electrode material layer that contains an electrode active material, a highly dielectric oxide solid and an electrolyte solution, wherein the electrolyte solution has a solvent with an average molecular weight of 110 or more, a flash point of 21 ℃ or more and a viscosity of 3.0mPa.s or more.

Description

Electrode for lithium ion secondary battery and lithium ion secondary battery
Technical Field
The present invention relates to an electrode for a lithium ion secondary battery and a lithium ion secondary battery using the same.
Background
Conventionally, lithium ion secondary batteries have been widely used as secondary batteries having a high energy density. A lithium ion secondary battery using a liquid as an electrolyte has a structure in which a separator is present between a positive electrode and a negative electrode, and an electrolyte (electrolytic solution) filled with the liquid is filled.
Such a lithium ion secondary battery generally has poor thermal stability because an organic solvent is used as a liquid electrolyte. In this regard, the following technique is proposed: by adding a small amount of a fluorine-based solvent having a flash point of 150 ℃ or higher to the electrolyte solution, the occurrence of a burst or fire due to a needle puncture can be suppressed without increasing the resistance of the battery (see patent document 1).
However, if the amount of the fluorine-based solvent to be added is increased in order to improve thermal stability, the durability is deteriorated, and as a result, the safety and durability cannot be sufficiently satisfied at the same time.
[ Prior art documents ]
(patent document)
Patent document 1: japanese patent laid-open No. 2001-060464
Disclosure of Invention
[ problems to be solved by the invention ]
The present invention has been made in view of the above-mentioned background art, and an object thereof is to provide an electrode for a lithium ion secondary battery that can satisfy both thermal stability and durability, and a lithium ion secondary battery using the same.
[ means for solving problems ]
The present inventors have conducted extensive studies and found that the above problems can be solved if a specific electrolyte solution and highly dielectric solid particles are present in an electrode material layer, and thus the present invention has been completed.
That is, the present invention provides an electrode for a lithium ion secondary battery, comprising an electrode mixture layer containing an electrode active material, a highly dielectric oxide solid, and an electrolyte solution, wherein the electrolyte solution has a solvent having an average molecular weight of 110 or more, a flash point of 21 ℃ or more, and a viscosity of 3.0mpa.s or more.
Alternatively, the highly dielectric oxide solid and the electrolytic solution may be disposed in a gap between the electrode active materials.
Alternatively, in a cross-sectional view of the electrode for a lithium ion secondary battery, a ratio of a cross-sectional area of the highly dielectric oxide solid to a cross-sectional area of the entire gap is 1 to 22%.
Optionally, the highly dielectric oxide solid is an oxide solid electrolyte.
Optionally, the oxide solid electrolyte is selected from Li 7 La 3 Zr 2 O 12 (LLZO)、Li 6.75 La 3 Zr 1.75 Ta 0.25 O 12 (LLZTO)、Li 0.33 La 0.56 TiO 3 (LLTO)、Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 (LATP) and Li 1.6 Al 0.6 Ge 1.4 (PO 4 ) 3 (lag).
Alternatively, the volume filling rate of the electrode active material is 60% or more with respect to the volume of the entire electrode material constituting the electrode.
Optionally, the thickness of the electrode material layer is 40 μm or more.
Alternatively, the electrode for a lithium ion secondary battery is a positive electrode.
Alternatively, the electrode for a lithium ion secondary battery is a negative electrode.
In addition, another aspect of the present invention provides a lithium ion secondary battery including the electrode for a lithium ion secondary battery and an electrolyte solution.
(Effect of the invention)
According to the electrode for a lithium ion secondary battery of the present invention, a lithium ion secondary battery satisfying both thermal stability and durability can be realized.
Drawings
Fig. 1 is a view showing one embodiment of a lithium ion secondary battery of the present invention.
Detailed Description
Hereinafter, embodiments of the present invention will be described. Further, the present invention is not limited to the following embodiments.
< electrode for lithium ion Secondary Battery >
The electrode for a lithium ion secondary battery of the present invention has an electrode composite layer containing an electrode active material, a highly dielectric oxide solid and an electrolyte, and the electrolyte contained in the electrode composite layer is the following electrolyte: the solvent has an average molecular weight of 110 or more, a flash point of 21 ℃ or more, and a viscosity of 3.0mPa.s or more.
The electrode for a lithium ion secondary battery of the present invention may be a positive electrode for a lithium ion secondary battery or a negative electrode for a lithium ion secondary battery.
The structure of the electrode for a lithium ion secondary battery of the present invention is not particularly limited, and examples thereof include the following: an electrode material layer composed of an electrode material mixture containing an electrode active material and a highly dielectric oxide solid is laminated on an electrode current collector, and the electrode material layer is impregnated with an electrolyte.
[ Current collector ]
The electrode current collector in the electrode for a lithium ion secondary battery of the present invention is not particularly limited, and a known current collector used in a lithium ion secondary battery may be used.
Examples of the material of the positive electrode current collector include metal materials such as stainless steel (SUS), Ni, Cr, Au, Pt, Al, Fe, Ti, Zn, and Cu. Examples of the material of the negative electrode current collector include SUS, Ni, Cu, Ti, Al, calcined carbon, conductive polymer, conductive glass, and Al — Cd alloy.
Examples of the shape of the electrode current collector include foil, plate, and mesh. The thickness is not particularly limited, and may be, for example, 1 to 20 μm, and may be appropriately selected as needed.
[ electrode laminate layer ]
In the electrode for a lithium ion secondary battery of the present invention, the electrode mixture layer contains an electrode active material and a highly dielectric oxide solid as essential components. The electrode material layer may be formed on at least one surface of the current collector, or may be formed on both surfaces. It can be appropriately selected according to the kind and structure of the target lithium ion secondary battery.
The electrode material layer may optionally contain other components if it contains the electrode active material and the highly dielectric oxide solid, which are the components of the present invention, as essential components. Examples of the optional components include known components such as a conductive aid and a binder.
(thickness of electrode composite layer)
The thickness of the electrode material layer of the electrode for a lithium ion secondary battery of the present invention is not particularly limited, and is preferably 40 μm or more, for example. When the thickness is 40 μm or more and the volume filling rate of the electrode active material is 60% or more, the obtained electrode for a lithium ion secondary battery becomes a high-density electrode. Furthermore, the volume energy density of the prepared battery unit can reach more than 500 Wh/L.
[ electrolyte ]
In the electrode for a lithium ion secondary battery of the present invention, the average molecular weight, flash point, and viscosity of the solvent of the electrolyte solution disposed in the gaps between the electrode active material particles satisfy specific conditions.
The electrolyte used when forming a secondary battery using the electrode for a lithium ion secondary battery of the present invention may be the same as or different from the electrolyte disposed in the electrode for a lithium ion secondary battery of the present invention.
(solvent)
{ average molecular weight }
The average molecular weight of the solvent constituting the electrolyte solution contained in the electrode mixture layer of the electrode for a lithium ion secondary battery of the present invention is 110 or more. The average molecular weight is preferably 115 or more, more preferably 120 or more.
If the average molecular weight of the solvent constituting the electrolyte solution contained in the electrode composite layer is 110 or more, the flash point is 21 ℃ or more, and therefore the possibility of ignition at the time of occurrence of an abnormality is reduced.
In addition, as a method for preparing the average molecular weight in the above range, a method of mixing a desired amount of a relatively large molecular weight compound such as a carbonate solvent may be mentioned.
{ flash Point }
The flash point of a solvent constituting an electrolyte contained in an electrode mixture layer of an electrode for a lithium ion secondary battery of the present invention is 21 ℃ or higher. The flash point is more preferably 25 ℃ or higher.
If the flash point of the solvent constituting the electrolyte contained in the electrode composite layer is 21 ℃ or higher, a lithium ion secondary battery having excellent stability in a high-temperature environment can be produced.
In addition, as a method for preparing the flash point within the above range, a method of mixing a high flash point solvent is exemplified, and as the high flash point solvent, for example, t-butyl carbonate and the like are exemplified.
{ viscosity }
The viscosity of a solvent constituting an electrolyte solution contained in an electrode mixture layer of an electrode for a lithium ion secondary battery according to the present invention is 3.0mpa.s or more. The viscosity is more preferably 3.5mpa.s, and still more preferably 4.0mpa.s or more.
In general, if the viscosity of the solvent constituting the electrolyte solution contained in the electrode composite layer is increased to 3.0mpa.s or more, lithium ions are less likely to diffuse and the ionic conductivity is reduced. However, the electrode for a lithium ion secondary battery of the present invention is considered to have improved ion conductivity because not only the electrolytic solution but also the highly dielectric oxide solid exists in the gaps formed between the electrode active material particles. This can provide an electrode having excellent thermal stability, and can ensure the safety of a lithium ion secondary battery.
As a method for preparing the viscosity within the above range, for example, a method of appropriately mixing a solvent having a high viscosity such as EC or PC with a solvent having a low viscosity such as DMC or EMC may be mentioned.
{ kind }
As the solvent constituting the electrolytic solution contained in the electrode composite layer of the electrode for a lithium ion secondary battery of the present invention, a solvent forming a general nonaqueous electrolytic solution can be used. Examples thereof include cyclic carbonates having a cyclic structure such as Ethylene Carbonate (EC) and Propylene Carbonate (PC); chain carbonates such as dimethyl carbonate (DMC), Ethyl Methyl Carbonate (EMC), and diethyl carbonate (DEC).
In addition, a carbonate having a relatively large molecular weight such as benzylphenyl carbonate, bis (pentafluorophenyl) carbonate, bis (2-methoxyphenyl) carbonate, bis (pentafluorophenyl) carbonate or tert-butylphenyl carbonate can also be used.
Furthermore, fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC) obtained by partially fluorinating them may be used.
In addition, a known additive may be blended in the electrolyte solution, and examples of the additive include Vinylene Carbonate (VC), Vinyl Ethylene Carbonate (VEC), Propane Sultone (PS), fluoroethylene carbonate (FEC), and the like.
In addition, an ionic liquid may be contained as the electrolyte. Examples of the ionic liquid include pyrrolidinium, piperidinium, and imidazolium salts composed of a quaternary ammonium cation.
Generally, when the electrolyte contains a large amount of a low boiling point solvent such as a chain carbonate, the heat generation amount is large if the battery is overcharged. Therefore, in order to ensure sufficient safety, a protection circuit for preventing overcharge is provided, or a plurality of safety valves, current cutoff valves, and other protection mechanisms are used together, which not only complicates the battery manufacturing process, but also reduces the battery energy density.
On the other hand, when the electrolyte contains a large amount of a high boiling point solvent such as a cyclic carbonate or a long chain carbonate, the electrolyte is unevenly distributed during charge and discharge cycles, and the durability of the battery is lowered, although the safety is secured.
The electrolyte contained in the electrode material layer of the electrode for a lithium ion secondary battery according to the present invention has the following composition: the ratio of the cyclic carbonate as a solvent is increased and also the ratio of the carbonate having a larger molecular weight is increased. In the present invention, by allowing such an electrolytic solution to coexist with the highly dielectric oxide solid contained in the electrode material layer, the electrolyte solution is prevented from being non-uniform, and the ionic conductivity is improved, so that the battery safety can be improved without adversely affecting the durability.
In the electrolyte solution contained in the electrode for a lithium ion secondary battery of the present invention, the ratio of the cyclic carbonate is preferably 15 vol% or more and 50 vol% or less. More preferably 20 to 45 vol%, and particularly preferably 25 to 40 vol%.
In the electrolyte contained in the electrode for a lithium ion secondary battery of the present invention, the ratio of the carbonate having a relatively high molecular weight is preferably 0.01 vol% or more and 50 vol% or less. More preferably 0.05% by volume or more and 40% by volume or less, and particularly preferably 0.1% by volume or more and 30% by volume or less.
In the electrolyte solution contained in the electrode for a lithium ion secondary battery of the present invention, the ratio of the chain carbonate is preferably 1 vol% or more and 80 vol% or less. More preferably 10% by volume or more and 75% by volume or less, and particularly preferably 20% by volume or more and 70% by volume or less.
(lithium salt)
In the electrode for a lithium ion secondary battery of the present invention, the lithium salt contained in the electrolyte disposed in the gaps between the electrode active material particles is not particularly limited, and examples thereof include LiPF 6 、LiBF 4 、LiClO 4 、LiN(SO 2 CF 3 )、LiN(SO 2 C 2 F 5 ) 2 、LiCF 3 SO 3 And the like. Among them, LiPF having high ionic conductivity and high dissociation degree is preferable 6 、LiBF 4 Or mixtures thereof.
The concentration of the lithium salt contained in the electrolyte disposed in the gaps between the electrode active material particles is in the range of 0.5 to 3.0 mol/L. When the amount is less than 0.5mol/L, the ionic conductivity is lowered, while when the amount exceeds 3.0mol/L, the viscosity is high and the ionic conductivity is low, so that it is difficult to sufficiently obtain the effect of the solid oxide.
In the present invention, the concentration of the lithium salt contained in the electrolyte solution disposed in the gaps between the electrode active material particles is preferably in the range of 1.0 to 3.0mol/L, and most preferably in the range of 1.2 to 2.2mol/L for the purpose of improving the output performance after the durability.
In general, when the concentration of the lithium salt in the electrolytic solution is high, the viscosity of the electrolytic solution becomes high, and thus the permeability of the electrolytic solution to the electrode decreases. However, in the electrode for a lithium ion secondary battery of the present invention, not only the electrolytic solution but also the highly dielectric oxide solid exists in the gap formed between the electrode active material particles, and thus the permeability of the electrolytic solution is improved.
In addition, generally, in the case where the concentration of the lithium salt in the electrolytic solution is high, association of lithium ions with anions occurs, resulting in a tendency for the ion conductivity to decrease. However, the electrode for a lithium ion secondary battery of the present invention is considered to have improved ion conductivity because not only the electrolytic solution but also the highly dielectric oxide solid exists in the gap formed between the electrode active material particles.
Therefore, in the electrode for a lithium ion secondary battery of the present invention, the electrolyte solution disposed in the gaps between the electrode active material particles can be applied with a concentration higher than that of the lithium salt in the electrolyte solution applied to the conventional lithium ion secondary battery. Even when an electrolyte having a high concentration is used, the time for which the electrolyte is impregnated into the electrode is short, so that productivity can be improved, and a battery having a high initial capacity can be obtained.
[ electrode active Material ]
The electrode active material contained in the electrode for a lithium ion secondary battery of the present invention is not particularly limited as long as it can occlude and release lithium ions, and a known material can be used as the electrode active material for a lithium ion secondary battery.
(Positive electrode active Material)
When the electrode for a lithium ion secondary battery of the present invention is a positive electrode for a lithium ion secondary battery, the positive electrode active material is not particularly limited, and examples thereof include LiCoO 2 、LiCoO 4 、LiMn 2 O 4 、LiNiO 2 、LiFePO 4 Lithium sulfide, sulfur, and the like. As the positive electrode active material, a material capable of exhibiting a higher potential than that of the negative electrode may be selected from materials capable of constituting the electrode.
(negative electrode active Material)
When the electrode for a lithium ion secondary battery of the present invention is a negative electrode for a lithium ion secondary battery, examples of the negative electrode active material include metallic lithium, lithium alloys, metal oxides, metal sulfides, metal nitrides, silicon oxide, silicon, carbon materials such as graphite, and the like. As the negative electrode active material, a material capable of exhibiting a lower potential than that of the positive electrode may be selected from materials capable of constituting the electrode.
(volume filling ratio of electrode active Material)
The electrode for a lithium ion secondary battery of the present invention preferably has an electrode active material volume filling rate of 60% or more with respect to the volume of the entire electrode material layer. If the volume filling rate of the electrode active material is 60% or more, the proportion of the gaps formed between the electrode active material particles is less than 40% with respect to the volume of the entire electrode composite layer. Therefore, an electrode for a lithium ion secondary battery having a small gap ratio can be obtained, and an electrode having a large volumetric energy density can be obtained. If the volume filling rate of the electrode active material is 60% or more, for example, a high volumetric energy density of 500Wh/L or more can be achieved in the battery cell.
In the present invention, the volume filling rate of the electrode active material with respect to the volume of the entire electrode material constituting the electrode is more preferably 65% or more, and most preferably 70% or more.
[ highly dielectric oxide solid ]
The highly dielectric oxide solid contained in the electrode for a lithium ion secondary battery of the present invention is not particularly limited as long as it is an oxide having a high dielectric property. In general, the dielectric constant of solid particles pulverized from a crystalline state changes from the original crystalline state, and the dielectric constant decreases. Therefore, the highly dielectric oxide solid used in the present invention is preferably a powder obtained by pulverizing the highly dielectric oxide solid in a state where the high dielectric state can be maintained as much as possible.
(relative permittivity of powder)
The relative dielectric constant of the powder of the highly dielectric oxide solid used in the present invention is preferably 10 or more, and more preferably 20 or more. When the powder relative permittivity is 10 or more, the increase in internal resistance can be suppressed even when charge and discharge cycles are repeated, and a lithium ion secondary battery having excellent durability against charge and discharge cycles can be sufficiently realized.
The term "powder relative dielectric constant" as used herein means a value determined as follows.
(method of measuring relative dielectric constant of powder)
The powder is introduced into a tablet forming machine having a diameter (R) of 38mm for measurement, and is compressed by a hydraulic press so that the thickness (d) thereof becomes 1 to 2mm, thereby forming a green compact. The molding conditions of the green compact were: relative density of powder (D) powder ) The weight density of the powder/the true specific gravity of the dielectric medium x 100 was 40% or more, and the electrostatic capacity C of the molded article at 25 ℃ and 1kHz was measured by an auto-balance bridge method using an LCR apparatus total And calculating the relative dielectric constant ε of the powder compact total . To obtain the dielectric constant epsilon of a real volume part from the relative dielectric constant of the obtained powder compact power Dielectric constant ε of vacuum 0 Set to 8.854 × 10 -12 The relative dielectric constant ε of air air Assuming that 1 is used, the "powder relative dielectric constant ε" is calculated by the following equations (1) to (3) power ”。
Contact area between powder compact and electrode (R/2) 2 ×π (1)
C total ＝ε total ×ε 0 ×(A/d) (2)
ε total ＝ε powder ×D powder +ε air ×(1-D powder ) (3)
(particle diameter)
The particle diameter of the highly dielectric oxide solid is not particularly limited, but is preferably 0.1 μm or more and about 10 μm or less of the particle size of the active material. If the solid particle diameter of the highly dielectric oxide is too large, the filling ratio of the active material in the electrode is inhibited from increasing.
(preparation of highly dielectric oxide solid)
In the electrode material layer of the electrode for a lithium ion secondary battery of the present invention, the highly dielectric oxide solid is preferably disposed in the gap between the electrode active materials. The gaps between the particles formed in the electrode active material can be controlled by the filling rate of the electrode active material, and are related to the density of the electrode composite layer. Further, a resin binder as a binder, a carbon material as a conductive aid for providing electron conductivity, and the like may be disposed in the gaps between the particles of the electrode active material.
By disposing the highly dielectric oxide solid in the gaps between the particles of the electrode active material, the electrode for a lithium ion secondary battery of the present invention can suppress decrease in diffusion of lithium ions in the electrode and increase in resistance, and can realize an electrode having a high packing density of the electrode active material. As a result, it is possible to realize a lithium ion secondary battery capable of suppressing a decrease in output due to repeated charge and discharge even when the volumetric energy density is high and the amount of electrolyte held by the electrode is small.
In addition, the electrolyte permeability of the electrode for a lithium ion secondary battery of the present invention is improved by disposing the highly dielectric oxide solid in the gaps between the particles of the electrode active material. As a result, the uniformity of the electrolyte held in the electrode is improved. Therefore, a Solid Electrolyte Interface (SEI) film can be uniformly formed on the negative electrode, and lithium electrodeposition can be suppressed. Further, the impregnation time of the electrolyte solution into the electrode can be shortened, and the productivity can be improved.
Further, by disposing the highly dielectric oxide solid in the gaps between the particles of the electrode active material, the electrode for a lithium ion secondary battery of the present invention can suppress the association of lithium ions and anions by a dielectric effect. As a result, for example, even when an electrolytic solution containing a high concentration of lithium salt is used, the effect of reducing the resistance can be exhibited.
In addition, since the highly dielectric oxide solid is previously mixed in the electrode mix slurry for forming the electrode mix layer, the highly dielectric oxide solid can be easily arranged between the electrode active material particles in the formed electrode mix layer, and the highly dielectric oxide solid can be easily arranged substantially uniformly over the entire electrode mix layer. Further, if a highly dielectric oxide solid is attached to a conductive assistant, a binder, and the like in advance and then mixed with an electrode active material to prepare an electrode mix slurry, dielectric solid powder can be arranged in a more uniform state in gaps between particles of the electrode active material.
(occupancy of cross-sectional area of highly dielectric oxide solid in gap portion)
In the electrode for a lithium ion secondary battery of the present invention, the occupancy ratio of the highly dielectric oxide solid in the gaps between the electrode active material particles is preferably: in the cross-sectional observation of the electrode for a lithium ion secondary battery, the ratio of the cross-sectional area of the highly dielectric oxide solid to the cross-sectional area of the entire gap is in the range of 1 to 22%. When the amount is in this range, the effects of reducing the resistance and improving the durability can be obtained at the same time.
As described above, the gap in the present invention is a region other than the region occupied by the active material in the electrode mixture layer, and a resin binder as a binder, a carbon material for providing electron conductivity, and the like may be disposed in the gap. When the occupancy rate of the highly dielectric oxide solid in the gap portion was determined, the electrode for a lithium ion secondary battery was observed in cross section. The cross-sectional observation was performed according to the following procedure.
(method of Cross-section observation)
-making the cross section of the electrode composite layer by using an ion milling method and observing by using a Scanning Electron Microscope (SEM).
The imaging range of the cross-sectional SEM is selected to be about 80% or more with respect to the thickness direction (vertical direction) of the electrode composite layer.
The imaging magnification is set to about 5000 to 10000 times, and the imaging is performed as a plurality of images in a divided manner.
Images in the plane direction (left-right direction) are taken as in the up-down direction.
And binarizing the brightness of the reflected electron image by combining the obtained images, and deriving the area occupancy rate of each component forming the electrode material according to the brightness distribution curve.
The active material region and the oxide solid region are set with respect to the area occupancy, and the other dark portion is defined as a remaining space. In the remaining space, a resin binder, a conductive aid, and the like are present, and pores containing an electrolytic solution are also contained.
The reason why the cross-sectional area occupancy of the highly dielectric oxide solid in the gap portion is preferably in the range of 1 to 22% is because of the dielectric constant of the highly dielectric oxide solid itself. Specifically, if the dielectric constant of the highly dielectric oxide solid is high, the influence on the electrolytic solution is large, and therefore the preferred cross-sectional area occupancy of the highly dielectric oxide solid is close to 1%. Conversely, if the dielectric constant of the highly dielectric oxide solid is small, the preferred cross-sectional area occupancy of the highly dielectric oxide solid approaches 22%.
When the occupancy of the cross-sectional area of the highly dielectric oxide solid is less than 1%, the dielectric effect of the highly dielectric oxide solid is reduced, and only the same effect as that of the conventional electrolyte can be obtained. On the other hand, when the occupancy of the cross-sectional area of the highly dielectric oxide solid is more than 22%, the electrolyte in the gap portion is relatively decreased, resulting in insufficient liquid, and the lithium ion transport path is decreased, increasing the internal resistance.
(kind of highly dielectric oxide solid)
The highly dielectric oxide solid is not particularly limited as long as it is an oxide having a high dielectric property, but is preferably an oxide solid electrolyte. An oxide solid electrolyte enables to produce an inexpensive crystal and is excellent in electrochemical oxidation resistance and reduction resistance. In addition, since the true specific gravity of the oxide solid electrolyte is small, the increase in the weight of the electrode can be suppressed.
Further, the highly dielectric oxide solid is preferably an oxide solid electrolyte having lithium ion conductivity. If the electrolyte is a highly dielectric oxide solid electrolyte having lithium ion conductivity, the output of the resulting lithium ion secondary battery at low temperature can be further improved. In addition, an electrode for a lithium ion secondary battery having excellent electrochemical oxidation resistance and reduction resistance can be produced at a relatively low cost.
Examples of the highly dielectric oxide solid include: BaTiO 2 3 、Ba x Sr 1-x TiO 3 (X＝0.4～0.8)、BaZr x Ti 1-x O 3 (X＝0.2～0.5)、KNbO 3 And the like having a perovskite type crystal structure; SrBi 2 Ta 2 O 9 、SrBi 2 Nb 2 O 9 And the like, a composite metal oxide having a layered perovskite crystal structure containing bismuth.
Further, the highly dielectric oxide solid is preferably a substance having lithium ion conductivity, and is more preferably selected from the group consisting of Li 7 La 3 Zr 2 O 12 (LLZO)、Li 6.75 La 3 Zr 1.75 Ta 0.25 O 12 (LLZTO)、Li 0.33 La 0.56 TiO 3 (LLTO)、Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 (LATP) and Li 1.6 Al 0.6 Ge 1.4 (PO 4 ) 3 (lag).
(amount of solid highly dielectric oxide)
The amount of the highly dielectric oxide solid to be mixed in the electrode mix layer is preferably in the range of 0.1 to 5 mass%, more preferably in the range of 0.25 to 4 mass%, and particularly preferably in the range of 0.5 to 3 mass% of the total mass of the electrode mix layer. When the amount is in the range of 0.1 to 5% by mass, the effects of reducing the resistance and improving the durability can be obtained at the same time.
< method for producing electrode for lithium ion Secondary Battery >
The method for producing the electrode for a lithium ion secondary battery of the present invention is not particularly limited, and conventional methods in the art can be applied. For example, the following methods can be cited: an electrode material slurry containing an electrode active material and a highly dielectric oxide solid as essential components is applied to an electrode current collector, dried, rolled, and impregnated with an electrolyte. At this time, by changing the pressing pressure of the pressure delay, the volume filling rate of the electrode active material (i.e., the ratio of the gaps formed between the electrode active material particles) can be controlled.
As a method of applying the electrode slurry to the electrode current collector, a known method can be applied. Examples thereof include roll coating such as an applicator roll, screen coating, blade coating, spin coating, and bar coating.
< lithium ion Secondary Battery >
The lithium ion secondary battery of the present invention includes the electrode for a lithium ion secondary battery of the present invention and an electrolyte solution. In the lithium ion secondary battery of the present invention, the electrode for a lithium ion secondary battery of the present invention may be a positive electrode or a negative electrode, and both the positive electrode and the negative electrode may be the electrode for a lithium ion secondary battery of the present invention.
Fig. 1 illustrates one embodiment of a lithium ion secondary battery according to the present invention. The lithium-ion secondary battery 10 shown in fig. 1 includes: a positive electrode 4 including a positive electrode mixture layer 3 formed on a positive electrode current collector 2; a negative electrode 7 including a negative electrode mixture layer 6 formed on a negative electrode current collector 5; a separator 8 electrically insulating the positive electrode 4 and the negative electrode 7; an electrolyte 9; the container 10 accommodates the positive electrode 4, the negative electrode 7, the separator 8, and the electrolyte 9.
In the container 1, the positive electrode mixture layer 3 and the negative electrode mixture layer 6 are opposed to each other with the separator 8 interposed therebetween, and the electrolyte solution 9 is stored below the positive electrode mixture layer 3 and the negative electrode mixture layer 6. The end of the separator 8 is immersed in the electrolyte 9. The positive electrode 4 or the negative electrode 7, or both of them are the electrode for a lithium ion secondary battery of the present invention, and include an electrode active material, a highly dielectric oxide solid, and an electrolytic solution, and the highly dielectric oxide solid and the electrolytic solution are disposed in gaps between particles formed in the electrode active material.
[ Positive and negative electrodes ]
In the lithium ion secondary battery of the present invention, either the positive electrode or the negative electrode, or both the positive electrode and the negative electrode are used as the electrode for the lithium ion secondary battery of the present invention. When only the positive electrode is used as the electrode for a lithium ion secondary battery of the present invention, a metal, a carbon material, or the like as a negative electrode active material may be used as it is as a sheet as a negative electrode.
[ electrolyte ]
The electrolyte solution to be used in the lithium ion secondary battery of the present invention is not particularly limited, and a known electrolyte solution can be used as the electrolyte solution of the lithium ion secondary battery. The electrolyte used in forming the lithium ion secondary battery may be the same as or different from the electrolyte disposed in the electrode for a lithium ion secondary battery of the present invention.
< method for producing lithium ion Secondary Battery >
The method for producing the lithium ion secondary battery of the present invention is not particularly limited, and conventional methods in the art can be applied.
[ examples ]
The present invention will be described in further detail with reference to examples and the like, but the present invention is not limited thereto.
< example 1>
[ preparation of Positive electrode ]
Acetylene black as a conductive aid and Li as an oxide solid electrolyte 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 (LATP) and then dispersed by a rotation-revolution agitator to obtain a mixture. Then, polyvinylidene fluoride (PVDF) as a binder and LiNi as a positive electrode active material were added to the obtained mixture 0.6 Co 0.2 Mn 0.2 O 2 (NCM622, D50 ═ 12 μm), and dispersion treatment was performed with a planetary mixer to obtain a mixture for a positive electrode material. In addition, the ratio of each component in the mixture for a positive electrode material is, in terms of mass ratio, positive electrode active material: LATP: conductive auxiliary agent: the resin binder (PVDF) was mixed so that the amount of the LATP added was 2 parts by weight per 100 parts by weight of the positive electrode material mixture, that is, 92.1:2:4.1: 1.8. Then, the obtained mixture for a positive electrode material was dispersed in N-methyl-2-pyrrolidineKetone (NMP) to prepare a positive electrode mixture slurry.
An aluminum foil having a thickness of 12 μm was prepared as a current collector, and the prepared positive electrode mixture slurry was applied to one surface of the current collector, dried at 120 ℃ for 10 minutes, pressed at a line pressure of 1t/cm by a roll press, and then dried in vacuum at 120 ℃ to prepare a positive electrode for a lithium ion secondary battery. The manufactured positive electrode was used after being pressed to a thickness of 30mm × 40 mm.
The thickness of the electrode material layer in the obtained positive electrode for a lithium ion secondary battery was 68 μm. The volume filling rate of the electrode active material with respect to the volume of the entire electrode material was 65.9%. The measurement method is described below.
(method of measuring thickness of electrode mixture layer)
The current collecting foil and the electrode material layer of the positive electrode for a lithium ion secondary battery obtained were integrated. The thicknesses of the collector foil portions were measured together with a thickness gauge, and the thickness of the electrode material layer was determined by subtracting the thickness of the collector foil portion.
(method of determining volume filling ratio of electrode active material to volume of the entire electrode material)
After the positive electrode for a lithium ion secondary battery was produced, the dry weight (unit area weight) of the electrode material layer was measured in advance, and the electrode material density was determined from the thickness of the electrode after pressing. According to the weight ratio and true specific gravity (g/cm) of each component constituting the electrode 3 ) The volume occupied by each component in the electrode material was determined, and the volume filling ratio of the electrode active material to the entire components was calculated. In addition, the positive electrode active material used in this example had a true specific gravity of 4.73g/cm 3 。
[ production of negative electrode ]
Sodium carboxymethylcellulose (CMC) as a binder and acetylene black as a conductive aid were mixed and dispersed by a planetary mixer to obtain a mixture. Artificial graphite (AG, D50 ═ 12 μm) as a negative electrode active material was mixed in the obtained mixture, and dispersion treatment was performed again by a planetary mixer to obtain a negative electrode material mixture. Then, the obtained mixture for a negative electrode mixture was dispersed in N-methyl-2-pyrrolidone (NMP), and styrene-butadiene rubber (SBR) as a binder was added thereto in a mass ratio of a negative electrode active material: conductive auxiliary agent: styrene Butadiene Rubber (SBR): a negative electrode mixture slurry was prepared so that the binder (CMC) was 96.5:1:1.5: 1.
A copper foil having a thickness of 12 μm was prepared as a current collector, and the prepared negative electrode mixture slurry was applied to one surface of the current collector, dried at 100 ℃ for 10 minutes, pressed at a line pressure of 1t/cm by a roll press, and then dried in vacuum at 100 ℃ to prepare a negative electrode for a lithium ion secondary battery. The produced negative electrode was used after being press-worked to 34mm × 44 mm.
The thickness of the electrode mixture layer was determined for the obtained negative electrode for a lithium ion secondary battery by the same method as that for the positive electrode. The thickness was 77 μm.
[ production of lithium ion Secondary Battery ]
A nonwoven fabric (thickness: 20 μm) of a polypropylene/polyethylene/polypropylene three-layer laminate was prepared as a separator. The aluminum laminate sheet for secondary batteries (manufactured by japan printing company, da) was heat-sealed to be processed into a pouch shape, and then the positive electrode, the separator, and the negative electrode prepared above were laminated and inserted therein.
(electrolyte)
As the electrolyte, LiPF was used so as to be 1.0mol/L 6 And a solution obtained by mixing ethylene carbonate, Ethyl Methyl Carbonate (EMC) and bis (pentafluorophenyl) carbonate in a volume ratio of 30:67.5: 2.5.
To the thus-prepared pouch, in which the positive electrode, the separator and the negative electrode were stacked and inserted, 0.128g (a volume amount of 120% relative to the interstitial volume) of the above-prepared electrolyte was added to prepare a lithium ion secondary battery.
The occupancy ratio of the dielectric oxide solid in the entire gap cross-sectional area of the electrode of the obtained lithium ion secondary battery was determined by the following method. The result was 11.6%.
(method of determining the occupancy of the cross-sectional area of the highly dielectric oxide solid relative to the cross-sectional area of the entire gap)
(1) And (3) cutting the cross section of the electrode by using an ion milling device for the positive electrode composite material layer or the negative electrode composite material layer to prepare a cross section sample of the electrode composite material layer.
(2) The image was taken with a field emission scanning electron microscope (FE-SEM) with an oversubscription voltage of 3kV, an imaging magnification of 5000 to 10000 times, and an image size of 1280 × 960. The element distribution of the cross-sectional sample was confirmed by reflection electron image and energy dispersive X-ray spectroscopy (EDX).
(3) The electrode active material particles, the highly dielectric oxide solid particles and other regions were divided by binarizing a reflection electron image of a cross-sectional sample, creating a luminance distribution curve, and differentiating the obtained curve to find an inflection point.
(4) The sectional area occupancy of the electrode active material particles, the sectional area occupancy of the highly dielectric oxide solid particles, and the sectional area occupancy (remaining space) of the other region are derived from the set division conditions.
(5) The operations (1) to (4) were carried out for a total of 8 positions in the vertical direction 3 and the horizontal direction 5 of the cross-sectional sample, and the average value of the occupation ratios of the cross-sectional areas of the highly dielectric oxide solid particles was defined as the occupation ratio of the cross-sectional area of the highly dielectric oxide solid with respect to the cross-sectional area of the entire gap.
When the sectional area occupancy is calculated, the sectional area occupancy a of the electrode active material particles, the sectional area occupancy B of the highly dielectric oxide solid particles, and the sectional area occupancy C of the remaining space, which is the other region, are determined. The occupancy of the cross-sectional area of the highly dielectric oxide solid particles with respect to the entire gap cross-sectional area is set to% of the ratio of the cross-sectional area occupancy B of the highly dielectric oxide solid particles to the sum of the cross-sectional area occupancy B of the highly dielectric oxide solid particles and the cross-sectional area occupancy C of the residual space (% ((B/(B + C) × 100).
< examples 2 to 3, comparative example 2>
A lithium ion secondary battery was produced in the same manner as in example 1, except that the composition of the electrolytic solution was changed as shown in table 1.
< comparative examples 1 and 3>
A lithium ion secondary battery was produced in the same manner as in example 1, except that LATP as an oxide solid electrolyte was not added to the positive electrode, and the composition of the electrolytic solution disposed in the gaps formed between the positive electrode active material particles was changed as shown in table 1.
< evaluation >
The lithium ion secondary batteries obtained in examples and comparative examples were evaluated as follows.
[ initial discharge Capacity ]
The lithium ion secondary battery thus produced was left at a measurement temperature (25 ℃) for 1 hour, charged at a constant current of 0.33C to 4.2V, then charged at a constant voltage of 4.2V for 1 hour, left for 30 minutes, discharged at a discharge rate of 0.2C to 2.5V, and the initial discharge capacity was measured. The results are shown in Table 1.
[ initial cell resistance ]
The charge level (soc (state of charge)) of the lithium ion secondary battery after the initial discharge capacity measurement was adjusted to 50%. Next, pulse discharge was performed for 10 seconds with the C rate set to 0.2C, and the voltage at 10 seconds of discharge was measured. Then, a graph of voltage versus current at 0.2C discharge for 10 seconds was plotted with the horizontal axis as the current value and the vertical axis as the voltage. After leaving for 5 minutes, the SOC was restored to 50% by recharging, and then left for another 5 minutes.
Next, the above operation was performed for each C rate of 0.5C, 1C, 2C, 5C, and 10C, and a graph of voltage versus current at 10 seconds of discharge at each C rate was plotted. The slope of the approximate straight line obtained from each plot was used as the initial battery resistance of the lithium-ion secondary battery obtained in this example. The results are shown in Table 1.
[ discharge Capacity after durability ]
As a charge-discharge cycle durability test, after constant current charging was performed to 4.2V at 1C in a constant temperature bath at 45 ℃, constant current discharging was performed to 2.5V at a discharge rate of 2C, the operation was regarded as 1 cycle, and the operation was repeated for 500 cycles. After the completion of 500 cycles, the thermostat was set to 25 ℃ and left to stand in a state after 2.5V discharge for 24 hours, and then the discharge capacity after the endurance was measured in the same manner as the measurement of the initial discharge capacity. The results are shown in Table 12.
[ cell resistance after durability ]
Similarly to the measurement of the initial cell resistance, the lithium ion secondary battery after the measurement of the discharge capacity after endurance was charged and adjusted to 50% (soc (state of charge)), and the resistance of the battery after endurance was measured in the same manner as the measurement of the initial cell resistance, and the results are shown in table 1.
[ cell resistance increase Rate ]
The durable cell resistance relative to the initial cell resistance was obtained as the cell resistance increase rate. The results are shown in Table 1.
[ Capacity Retention ratio ]
The discharge capacity after endurance relative to the initial discharge capacity was obtained as a capacity retention rate. The results are shown in tables 1 and 2.
[ viscosity ]
The measurement was performed at a rotation speed of 30rpm in an environment of 20 ℃ by using a rotary viscometer.
[ average molecular weight of solvent constituting electrolyte ]
The average molecular weight was calculated from the volume ratio of each solvent according to the following specific gravity.
Ethylene Carbonate (EC): 1.03g/mL
Dimethyl carbonate (DMC): 1.07g/mL
Diethyl carbonate (DEC): 0.97g/mL
Ethyl Methyl Carbonate (EMC): 1.02g/mL
Bis (pentafluorophenyl) carbonate: 1.78g/mL
Tert-butyl phenyl carbonate: 1.05g/mL
Benzylphenyl carbonate: 1.16g/mL
[ flash Point ]
The measurement was carried out according to the standard of Japanese Industrial Standard (JIS) K-2265 using a Tager closed cup flash point tester (model: ATG-7, manufactured by TIANCH scientific machines Co., Ltd.).
[ Table 1]
Figure BDA0003808858200000171
Reference numerals
10 lithium ion secondary battery
1 Container
2 positive electrode current collector
3 positive electrode composite material layer
4 positive electrode
5 negative electrode collector
6 negative pole composite material layer
7 negative electrode
8 diaphragm
9 electrolyte

Claims (10)

1. An electrode for a lithium ion secondary battery having an electrode composite layer containing an electrode active material, a highly dielectric oxide solid and an electrolytic solution, wherein,
the solvent of the electrolyte has an average molecular weight of 110 or more, a flash point of 21 ℃ or more, and a viscosity of 3.0mPa.s or more.
2. The electrode for a lithium ion secondary battery according to claim 1, wherein the highly dielectric oxide solid and the electrolytic solution are disposed in a gap between the electrode active materials.
3. The electrode for a lithium ion secondary battery according to claim 2, wherein a ratio of a cross-sectional area of the highly dielectric oxide solid to a cross-sectional area of the entire gap is 1 to 22% in a cross-sectional view of the electrode for a lithium ion secondary battery.
4. The electrode for a lithium ion secondary battery according to any one of claims 1 to 3, wherein the highly dielectric oxide solid is an oxide solid electrolyte.
5. The electrode for a lithium ion secondary battery according to claim 4, wherein the oxide solid electrolyte is selected from Li 7 La 3 Zr 2 O 12 (LLZO)、Li 6.75 La 3 Zr 1.75 Ta 0.25 O 12 (LLZTO)、Li 0.33 La 0.56 TiO 3 (LLTO)、Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 (LATP) and Li 1.6 Al 0.6 Ge 1.4 (PO 4 ) 3 (lag).
6. The electrode for a lithium-ion secondary battery according to any one of claims 1 to 5, wherein a volume filling rate of the electrode active material is 60% or more with respect to a volume of the entire electrode composite layer.
7. The electrode for a lithium ion secondary battery according to any one of claims 1 to 6, wherein the thickness of the electrode mixture layer is 40 μm or more.
8. The electrode for a lithium ion secondary battery according to any one of claims 1 to 7, wherein the electrode for a lithium ion secondary battery is a positive electrode.
9. The electrode for a lithium ion secondary battery according to any one of claims 1 to 7, wherein the electrode for a lithium ion secondary battery is a negative electrode.
10. A lithium ion secondary battery comprising the electrode for a lithium ion secondary battery according to any one of claims 1 to 7 and an electrolyte.
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